The ability to evade apoptosis (programmed cell death) is one of six molecular traits common to all human cancers. Work carried out during the previous funding term of CA78810 focused on survivin, a member of the Inhibitor of Apoptosis (lAP) gene family, differentially expressed in cancer but not in normal tissues. These studies led to the identification of a multifunctional survivin pathway, acting at the interface between cell cycle progression and apoptosis inhibition, and required to prevent caspase activation in cells traversing mitosis. Recently, we found that survivin and other members of the lAP gene family comprise a new class of client proteins for the chaperone Hsp90. This involves a novel, high affinity recognition of the Baculovirus lAP Repeat, approximately70-amino acid zinc finger fold that is the hallmark of all lAP proteins, and the NH2-terminus domain of Hsp90. Interference with the Hsp90 recognition results in loss of expression of survivin and other lAP proteins and spontaneous apoptosis. The central hypothesis of this continuation application is that binding to Hsp90 preserves the stability of the lAP gene family and maintains an anti-apoptotic threshold in cancer cells. In the first specific aim, we will map the structure-function requirements of the survivin/IAP-Hsp90 complex, identify the complementary binding sites involved in this interaction, and define the reciprocal relationship between lAPs and Hsp90 client protein binding/function. In the second specific aim, we will characterize the dynamic assembly of a survivin/IAP-Hsp90 complex in vivo with respect to ubiquitin-dependent proteasomal destruction, association with regulatory co-chaperones, exposure to cellular stress, and participation in apoptosome formation. In the third specific aim, we will target the survivin-Hsp90 interaction as a novel anti-cancer approach and its impact on mitochondrial-dependent apoptosis, caspase activation and interference with tumor growth, in vivo. The overall proposal is in full scientific continuity with the long-term objectives of CA78810 on the role of survivin as a master switch for cell death/viability in cancer. The cellular and molecular dissection of a survivin/IAP-Hsp90 complex, as proposed in the present continuation application will provide critical insights into novel anti-cancer strategies aimed at interfering with the general survival machinery of tumor cells.